Slip-form pavers have been widely used to pave concrete roads and generally comprise a tractor unit supported on two or four track assemblies with propulsion means, steering means and elevation means. From this tractor and usually at or near its midpoint is suspended a set of cross members consisting of a concrete spreading auger, a strike-off, internal vibrator for consolidating the concrete, a tamper bar and a screed or paving pan (sometimes called an extrusion meter). These latter elements will sometimes be referred to as a paving tool. On each side of the tool and located inside the traction means are the side forms which create the side edges of the ultimate concrete slab formed on the roadway as the machine moves progressively forward.
With the paving tool located near the longitudinal pivot point of the propulsion means, no steering problems are encountered should the machine be turned in either direction to provide the desired curve in the pathway of the road.
Development in the formulation of concrete mixtures has now made it possible to produce a concrete which will allow traffic on the pavement within 12 to 24 hours after placement. Such development permits the use of concrete overlays on previous pavements instead of the usual asphalt coating. In areas of heavy traffic and multiple lanes, only one lane may be shut down for placement of an overlay. If the pavement slab may be laid down during the interval, the overlaid lane can be opened for traffic the following day.
If the propulsion means are to travel within the confines of the lane to be overlaid, the spreading and paving means must be disposed behind and extend to a width at least as great as the outside width of the traction means. This has been done with asphalt paving machines in which the ends of the screed are supported by arms extending rearwardly from the tractor. Even the disposition of a spreading auger forwardly of the screed and also extending behind the traction members on the paver does not create a serious steering problem because of the close coupling of the units and the proximity of the screed to the center of turning radius of the tractor.
The mounting of the paving tool of a slip-form paver for concrete or other material in a manner similar to that of an asphalt paver creates a serious problem due to the significant distance between the location of the paving tool elements and the longitudinal pivot point of the tractor. The rear units of the concrete paver will swing on a much larger radius than rearmost elements of the asphalt paver. The more the distance from the tracks to the rear elements of the slip-form paving pan, the greater the radius of the path followed by the latter and the greater the distortion of the path from the desired plan.